Plant–plant and plant–soil interactions play a key role in determining plant community structure and ecosystem function. However, the effects of global change on the interplay between co-occurring plants and soil microbes in successional communities are poorly understood. In this study, we investigated competition for nitrogen (N) between soil microorganisms, grass plants and establishing tree seedlings under factorial carbon dioxide (CO2) and N treatments. Fraxinus excelsior seedlings were germinated in the presence or absence of grass competition (Dactylis glomerata) at low (380 μmol mol−1) or high (645 μmol mol−1) CO2 and at two levels of N nutrition in a mesocosm experiment. Pulse 15N labelling was used to examine N partitioning among plant and soil compartments. Dactylis exerted a strong negative effect on Fraxinus biomass, N capture and 15N recovery irrespective of N and CO2 treatment. In contrast, the presence of Dactylis had a positive effect on the microbial N pool. Plant and soil responses to N treatment were of a greater magnitude compared with responses to elevated CO2, but the pattern of Fraxinus- and microbial-N pool response to N and CO2 varied depending on grass competition treatment. Within the Dactylis competition treatment, decreases in Fraxinus biomass in response to N were not mirrored by decreases in tree seedling N content, suggesting a shift from below- to above-ground competition. In the Dactylis-sown pots, 15N recovery could be ranked Dactylis > microbial pool > Fraxinus in all N and CO2 treatment combinations. Inequalities between Fraxinus and soil microorganisms in terms of 15N recovery were exacerbated by N addition. Contrary to expectations, elevated CO2 did not increase plant–microbe competition. Nevertheless, microbial 15N recovery showed a small positive increase in the high CO2 treatment. Overall, elevated CO2 and N supply did not interact on plant/soil N partitioning. Our data suggest that the competitive balance between establishing tree seedlings and grass plants in an undisturbed sward is relatively insensitive to CO2 or N-induced modifications in N competition between plant and soil compartments.